Catalytically active particulate filter

ABSTRACT

The present invention relates to a particulate filter which comprises a wall-flow filter of length L and two different catalytically active coatings Y and Z, wherein the wall flow filter comprises channels E and A that extend in parallel between a first and a second end of the wall-flow filter and are separated by porous walls which form the surfaces OE and OA, respectively, and wherein the channels E are closed at the second end and the channels A are closed at the first end. The invention is characterized in that the coating Y is located in the channels E on the surfaces OE and the coating Z is located in the porous walls.

The present invention relates to a catalytically active particulatefilter that is particularly suitable for removing particles, carbonmonoxide, hydrocarbons and nitrogen oxides from the exhaust gas ofcombustion engines fueled by stoichiometric air-fuel mixture.

Exhaust gases from combustion engines, i.e. gasoline engines, fueled bystoichiometric air-fuel mixtures are cleaned in conventional methodswith the aid of three-way catalytic converters. Such catalyticconverters are capable of simultaneously converting the three majorgaseous pollutants of the engine, namely hydrocarbons, carbon monoxideand nitrogen oxides, into harmless components.

In addition to such gaseous pollutants, the exhaust gas from gasolineengines also contains extremely fine particles (PM) which result fromthe incomplete combustion of fuel and essentially consist of soot. Incontrast to particulate emission by diesel engines, the particles in theexhaust gas of stoichiometrically operated gasoline engines are verysmall and have an average particle size of less than 1 μm. Typicalparticle sizes range from 10 to 200 nm. Furthermore, the quantity ofparticles emitted is very low and ranges from 2 to 4 mg/km.

The European exhaust emission standard EU-6c is associated with aconversion of the limit value for such particles from the particle masslimit value to a more critical particle number limit value of 6×10¹¹/km(in the Worldwide Harmonized Light Vehicles Test Cycle—WLTP). Thiscreates a need for exhaust gas cleaning concepts for stoichiometricallyoperated combustion engines, which include effective equipment forremoving particles.

Wall-flow filters made of ceramic materials, such as silicon carbide,aluminum titanate and cordierite, have proven themselves in the field ofcleaning exhaust gases from lean-burn engines, i.e. in particular dieselengines. These are made up of a plurality of parallel channels formed byporous walls. The channels are alternately sealed at one of the two endsof the filter so that channels A, which are open at the first side ofthe filter and sealed at the second side of the filter, and channels B,which are sealed at the first side of the filter and open at the secondside of the filter, are formed. Exhaust gas flowing into, for example,channels A can only leave the filter via channels B and must flowthrough the porous walls between channels A and B for this purpose. Whenthe exhaust gas passes through the wall, the particles are retained andthe exhaust gas is cleaned.

The particles retained in this manner must then be burnt off or oxidizedin order to prevent a clogging of the filter or an unacceptable increasein the back pressure of the exhaust system.

For this purpose, the wall-flow filter is, for example, provided withcatalytically active coatings that reduce the ignition temperature ofsoot. Applying such coatings to the porous walls between the channels(so-called “on-wall coating”) or introducing them into the porous walls(so-called “in-wall coating”) is already known. EP 1 657 410 A2 alsoalready describes a combination of both coating types; that is, part ofthe catalytically active material is present in the porous walls andanother part is present on the porous walls.

The concept of removing particles from exhaust gas using wall-flowfilters has already been applied to the cleaning of exhaust gas fromcombustion engines operated with stoichiometric air-fuel mixtures; see,for example, EP 2042226 A2. According to its teaching, a wall-flowfilter comprises two layers arranged one above the other, wherein onecan be arranged in the porous wall and the other can be arranged on theporous wall.

DE 102011050788 A1 pursues a similar concept. There, the porous filterwalls contain a catalyst material of a three-way catalytic converter,while in addition a catalyst material of a three-way catalytic converteris applied to partial regions of the filter walls.

Other documents that describe filter substrates provided withcatalytically active coatings are EP 3205388 A1, EP 3207977 A1, EP3207978 A1, EP 3207987 A1, EP 3207989 A1. EP 3207990 A1 and EP 3162428A1.

There is still a need for catalytically active particulate filters thatcombine the functionalities of a particulate filter and a three-waycatalytic converter and at the same time adhere to the limits that willapply in the future.

The present invention relates to a particulate filter for removingparticles, carbon monoxide, hydrocarbons and nitrogen oxides from theexhaust gas of combustion engines operated with stoichiometric air-fuelmixtures, which filter comprises a wall-flow filter of length L and twocoatings Y and Z, wherein the wall-flow filter comprises channels E andA that extend in parallel between a first and a second end of thewall-flow filter and are separated by porous walls which form surfacesO_(E) and O_(A), respectively, and wherein the channels E are closed atthe second end and the channels A are closed at the first end,characterized in that the coating Y is located in the channels E on thesurfaces O_(E) and extends from the first end of the wall-flow filterover a length of 51 to 90% of the length L, and the coating Z is locatedin the porous walls and extends from the second end of the wall-flowfilter over a length of 60 to 100% of the length L.

The coatings Y and Z are three-way catalytically active, especially atoperating temperatures of 250 to 1100° C. They are preferably differentfrom each other, but both usually contain one or more noble metals fixedto one or more substrate materials and one or more oxygen storagecomponents. The coatings Y and Z may differ in the components theycontain. For example, they may differ in terms of the noble metals theycontain or the oxygen storage components they contain. However, they mayalso contain identical constituents. In the latter case, the coatings Yand Z may contain the components in equal or different amounts.

Platinum, palladium and rhodium are particularly suitable as noblemetals, wherein palladium, rhodium or palladium and rhodium arepreferred and palladium and rhodium are particularly preferred.

Based on the particulate filter according to the invention, theproportion of rhodium in the entire noble metal content is in particulargreater than or equal to 10% by weight.

The noble metals are usually used in quantities of 0.15 to 5 g/l basedon the volume of the wall-flow filter.

In the context of the invention, it may happen that some washcoat of thelayers Y penetrates into the surface pores of the wall-flow filterduring coating. According to the invention, however, this should beavoided as much as possible. Generally, the amount of washcoat whichpenetrates into the surface regions of the porous filter wall is <10%,more preferably <5% and most preferably <2%, based on the weight ofwashcoat used.

Since the coating Y is an on-wall coating in the present case, it has acertain elevation over the wall surface. However, the thickness of thetwo layers is generally between 5-250 μm, preferably 7.5-225 μm and mostpreferably between 10-200 μm.

All materials familiar to the person skilled in the art for this purposecan be considered as substrate materials for the noble metals. Suchmaterials are in particular metal oxides with a BET surface area of 30to 250 m²/g, preferably 100 to 200 m²/g (determined according to DIN66132—newest version on the date of application).

Particularly suitable substrate materials for the noble metals areselected from the series consisting of aluminum oxide, doped aluminumoxide, silicon oxide, titanium dioxide and mixed oxides of one or moreof these. Doped aluminum oxides are, for example, aluminum oxides dopedwith lanthanum oxide, zirconium oxide and/or titanium oxide.Lanthanum-stabilized aluminum oxide is advantageously used, whereinlanthanum is used in quantities of 1 to 10% by weight, preferably 3 to6% by weight, in each case calculated as La₂O₃ and based on the weightof the stabilized aluminum oxide.

Cerium/zirconium/rare earth metal mixed oxides are particularly suitableas oxygen storage components. The term “cerium/zirconium/rare earthmetal mixed oxide” within the meaning of the present invention excludesphysical mixtures of cerium oxide, zirconium oxide and rare earth oxide.Rather, “cerium/zirconium/rare earth metal mixed oxides” arecharacterized by a largely homogeneous, three-dimensional crystalstructure that is ideally free of phases of pure cerium oxide, zirconiumoxide or rare earth oxide. Depending on the manufacturing process,however, not completely homogeneous products may arise which cangenerally be used without any disadvantage.

In all other respects, the term “rare earth metal” or “rare earth metaloxide” within the meaning of the present invention does not includecerium or cerium oxide.

Lanthanum oxide, yttrium oxide, praseodymium oxide, neodymium oxideand/or samarium oxide can, for example, be considered as rare earthmetal oxides in the mixed cerium/zirconium/rare earth metal mixedoxides.

Lanthanum oxide, yttrium oxide and/or praseodymium oxide are preferred.Lanthanum oxide and/or yttrium oxide are particularly preferred, andlanthanum oxide and yttrium oxide, yttrium oxide and praseodymium oxide,and lanthanum oxide and praseodymium oxide are more particularlypreferred.

In embodiments of the present invention, the oxygen storage componentsare free from neodymium oxide.

In accordance with the invention, the cerium oxide to zirconium oxideratio in the cerium/zirconium/rare earth metal mixed oxides can varywithin wide limits. It can be, for example, 0.1 to 1.5, preferably 0.2to 1 or 0.3 to 0.5.

In embodiments of the present invention, the coating Y comprises anoxygen storage component with a cerium oxide content of 20 to 40% byweight, based on the weight of the oxygen storage component.

In embodiments of the present invention, the coating Z comprises anoxygen storage component with a cerium oxide content of 20 to 60% byweight, based on the weight of the oxygen storage component.

Lanthanum oxide-containing oxygen storage components have in particulara lanthanum oxide to cerium oxide mass ratio of 0.05 to 0.5.

The coatings Y and Z usually contain oxygen storage components inquantities from 15 to 120 g/l, based on the volume of the wall-flowfilter.

The mass ratio of substrate materials and oxygen storage components inthe coatings Y and Z is usually 0.3 to 1.5, for example 0.4 to 1.3.

In the embodiments of the present invention, one or both of the coatingsY and Z contain an alkaline earth compound, such as strontium oxide,barium oxide or barium sulfate. The amount of barium sulfate per coatingis, in particular, 2 to 20 g/l volume of the wall-flow filter.

Coating Z contains, in particular, strontium oxide or barium oxide.

In further embodiments of the present invention, one or both of thecoatings Y and Z contain additives, such as rare earth compounds, suchas lanthanum oxide, and/or binders, such as aluminum compounds. Suchadditives are used in quantities that can vary within wide limits andthat the person skilled in the art can determine in the specific case bysimple means.

In embodiments of the present invention, the coatings Y and Z aredifferent from each other, wherein, however, they both compriselanthanum-stabilized aluminum oxide, along with rhodium, palladium orpalladium and rhodium and an oxygen storage component comprisingzirconium oxide, cerium oxide, lanthanum oxide, and yttrium oxide and/orpraseodymium oxide.

In coating Z, the yttrium oxide content is in particular 5 to 15% byweight, based on the weight of the oxygen storage component. Thelanthanum oxide to yttrium oxide weight ratio is in particular 0.1 to 1,preferably 0.125-0.75 and most preferably 0.15-0.5.

In embodiments of the present invention, the yttrium oxide content inthe oxygen storage component of the coating Z is larger than or equal tothe yttrium oxide content in the oxygen storage component of the coatingY, based in each case on the weight of the respective oxygen storagecomponent.

In particular, the coating Z may comprise an additional oxygen storagecomponent containing zirconium oxide, cerium oxide, praseodymium oxideand lanthanum oxide.

In this case, the praseodymium oxide content in particular is 2 to 10%by weight, based on the weight of the oxygen storage component. Thelanthanum oxide to praseodymium oxide weight ratio is in particular 0.1to 2, preferably 0.125-1.7 and most preferably 0.15-1.5.

In embodiments of the present invention, in coating Z the zirconiumoxide content of the yttrium oxide-containing oxygen storage componentis greater than the zirconium oxide content of the praseodymiumoxide-containing oxygen storage component, in each case based on therespective oxygen storage component. In this embodiment in particular,it is advantageous if the weight ratio of Ce:Zr in the yttriumoxide-containing oxygen storage component (CeZr1) is smaller than theCe:Zr ratio in the praseodymium oxide-containing oxygen storagecomponent (CeZr2). In this case, the value of Ce:Zr1 is 0.1-1.0,preferably 0.15-0.75 and most preferably 0.2-0.6. By contrast, forCe:Zr2, values of 0.2-1.5, preferably 0.25-1.3 and most preferably0.3-1.1 are found.

In embodiments, the coatings Y and Z each comprise lanthanum-stabilizedaluminum oxide in quantities from 20 to 70% by weight, particularlypreferably 25 to 60% by weight, and the oxygen storage component inquantities from 25 to 80% by weight, particularly preferably 40 to 70%by weight, in each case based on the total weight of the coating Y or Z.

In embodiments of the present invention, in coating Y, the weight ratioof aluminum oxide to the oxygen storage component is at least 0.7.

In embodiments of the present invention, in coating Z, the weight ratioof aluminum oxide to the oxygen storage component is at least 0.3.

In embodiments of the present invention, the coating Y extends from thefirst end of the wall-flow filter over 51 to 90%, in particular 57 to85%, of the length L of the wall-flow filter. The load of the wall-flowfilter with coating Y preferably amounts to 33 to 125 g/l, based on thevolume of the wall-flow filter.

In embodiments of the present invention, the coating Z extends from thesecond end of the wall-flow filter over 51 to 100%, preferably over 57to 100%, more preferably over 90 to 100% of the length L of thewall-flow filter. The load of the wall-flow filter with coating Zpreferably amounts to 33 to 125 g/l, based on the volume of thewall-flow filter.

The total washcoat load of the particulate filter in accordance with theinvention is in particular 40 to 150 g/l, based on the volume ofwall-flow filter.

In embodiments of the present invention, the sum of the lengths ofcoating Y and coating Z is 110 to 180% of length L.

In embodiments of the present invention, neither coating Y nor coating Zcontains a zeolite or a molecular sieve.

In one embodiment of the present invention, the present inventionrelates to a particulate filter which comprises a wall-flow filter oflength L and two different coatings Y and Z, wherein the wall-flowfilter comprises channels E and A that extend in parallel between afirst and a second end of the wall-flow filter and are separated byporous walls forming surfaces O_(E) and O_(A), respectively, wherein thechannels E are closed at the second end and the channels A are closed atthe first end, wherein

Coating Y is located in the channels E on the surfaces O_(E) and extendsfrom the first end of the wall-flow filter over 57 to 65% of the lengthL and contains aluminum oxide in an amount of 35 to 60% by weight, basedon the total weight of the coating Y, palladium, rhodium or palladiumand rhodium and an oxygen storage component in an amount of 40 to 50% byweight, based on the total weight of the coating Y, wherein the oxygenstorage component comprises zirconium oxide, cerium oxide, lanthanumoxide and yttrium oxide or zirconium oxide, cerium oxide, lanthanumoxide and praseodymium oxide, and

Coating Z is located in the porous walls and extends from the second endof the wall-flow filter over 60 to 100% of the length L and containsaluminum oxide in an amount of 25 to 50% by weight, based on the totalweight of the coating, palladium, rhodium or palladium and rhodium andtwo oxygen storage components in a total amount of 50 to 80% by weight,based on the total weight of the coating Z, wherein one oxygen storagecomponent contains zirconium oxide, cerium oxide, lanthanum oxide andyttrium oxide and the other contains zirconium oxide, cerium oxide,lanthanum oxide and praseodymium oxide.

Wall-flow filters that can be used in accordance with the presentinvention are well-known and available on the market. They consist of,for example, silicon carbide, aluminum titanate or cordierite, and have,for example, a cell density of 200 to 400 cells per inch and usually awall thickness between 6 and 12 mil, or 0.1524 and 0.305 millimeters.

In the uncoated state, they have porosities of 50 to 80, in particular55 to 75%, for example. In the uncoated state, their average pore sizeis, for example, 10 to 25 micrometers.

Generally, the pores of the wall-flow filter are so-called open pores,that is, they have a connection to the channels. Furthermore, the poresare normally interconnected with one another. This enables, on the onehand, the easy coating of the inner pore surfaces and, on the otherhand, the easy passage of the exhaust gas through the porous walls ofthe wall-flow filter.

The particulate filter in accordance with the invention can be producedaccording to methods known to the person skilled in the art, for exampleby applying a coating suspension, which is usually called a washcoat, tothe wall-flow filter by means of one of the usual dip coating methods orpump and suction coating methods. Thermal post-treatment or calcinationusually follow.

The coatings Y and Z are obtained in separate and successive coatingsteps.

The person skilled in the art knows that the average pore size of thewall-flow filter and the average particle size of the catalyticallyactive materials must be matched to each other in order to achieve anon-wall coating or an in-wall coating. In the case of an in-wallcoating, the average particle size of the catalytically active materialsmust be small enough to penetrate the pores of the wall-flow filter. Incontrast, in the case of an on-wall coating, the average particle sizeof the catalytically active materials must be large enough not topenetrate the pores of the wall-flow filter.

In embodiments of the present invention, the coating suspensions for theproduction of the coatings Y are ground up to a particle sizedistribution of d₅₀=4 to 8 μm and d₉₉=22 to 16 μm.

In embodiments of the present invention, the coating suspensions for theproduction of coating Z are ground up to a particle size distribution ofd₅₀=1 to 2 μm and d₉₉=6 to 7 μm.

The particulate filter according to the invention is perfectly suitedfor removing particles, carbon monoxide, hydrocarbons and nitrogenoxides out of the exhaust gas of combustion engines operated withstoichiometric air/fuel mixture.

The present invention thus also relates to a method for removingparticles, carbon monoxide, hydrocarbons and nitrogen oxides from theexhaust gas of combustion engines operated with stoichiometric air/fuelmixture, characterized in that the exhaust gas is conducted through aparticulate filter according to the invention.

The exhaust gas can be conducted through a particulate filter accordingto the invention in such a way that it enters the particulate filterthrough channels E and leaves it again through channels A.

However, it is also possible for the exhaust gas to enter theparticulate filter through channels A and to leave it again throughchannels E.

FIG. 1 shows a particulate filter in accordance with the invention,comprising a wall-flow filter of length L (1) with channels E (2) andchannels A (3), which extend in parallel between a first end (4) and asecond end (5) of the wall-flow filter and are separated by porous walls(6) forming surfaces O_(E) (7) and O_(A) (8), respectively, and whereinchannels E (2) are closed at the second end (5) and channels A (3) areclosed at the first end (4). Coating Y (9) is located in the channels E(2) on the surfaces O_(E) (7) and coating Z (10) is located in theporous walls (6).

The invention is explained in more detail in the following examples.

COMPARATIVE EXAMPLE 1

Aluminum oxide stabilized with lanthanum oxide was suspended in waterwith a first oxygen storage component which comprised 40% by weightcerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxideand a second oxygen storage component which comprised 24% by weightcerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Bothoxygen storage components were used in equal parts. The weight ratio ofaluminum oxide and oxygen storage component was 30:70. The suspensionthus obtained was then mixed with a palladium nitrate solution and arhodium nitrate solution under constant stirring. The resulting coatingsuspension was used directly for coating a commercially availablewall-flow filter substrate, wherein the coating was introduced into theporous filter wall over 100% of the substrate length. The total load ofthis filter amounted to 100 g/l; the total noble metal load amounted to0.44 g/l with a ratio of palladium to rhodium of 8:3. The coated filterthus obtained was dried and then calcined. It is hereinafter referred toas VGPF 1.

EXAMPLE 1

a) Application of the in-Wall Coating:

Aluminum oxide stabilized with lanthanum oxide was suspended in waterwith a first oxygen storage component which comprised 40% by weightcerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxideand a second oxygen storage component which comprised 24% by weightcerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Bothoxygen storage components were used in equal parts. The weight ratio ofaluminum oxide and oxygen storage component was 30:70. The suspensionthus obtained was then mixed with a palladium nitrate solution and arhodium nitrate solution under constant stirring. The resulting coatingsuspension was used directly for coating a commercially availablewall-flow filter substrate, wherein the coating was introduced into theporous filter wall over 100% of the substrate length. The load of thisfilter amounted to 100 g/l; the noble metal load amounted to 0.34 g/lwith a ratio of palladium to rhodium of 16:3. The coated filter thusobtained was dried and then calcined.

b) Coating the Input Channels

Aluminum oxide stabilized with lanthanum oxide was suspended in waterwith an oxygen storage component which comprised 24% by weight ceriumoxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weightratio of aluminum oxide and oxygen storage component was 56:44. Thesuspension thus obtained was then mixed with a palladium nitratesolution and a rhodium nitrate solution under constant stirring. Theresulting coating suspension was used directly to coat the wall-flowfilter substrate obtained under a). wherein the filter walls of thesubstrate were coated in the input channels to a length of 38% of thefilter length. The load of the input channel amounted to 54 g/l; thenoble metal load amounted to 0.27 g/l with a ratio of palladium torhodium of 2.6:5. The coated filter thus obtained was dried and thencalcined. The total load of this filter thus amounted to 121 g/l; thetotal noble metal load amounted to 0.44 g/with a ratio of palladium torhodium of 8:3. It is hereinafter referred to as GPF 1.

Catalytic Characterization

The particulate filters VGPF1 and GPF1 were aged together in an enginetest bench aging process. This aging process consists of an overrun fuelcut-off aging process with an exhaust gas temperature of 950° C. beforethe catalyst inlet (maximum bed temperature of 1030° C.). The aging timewas 9.5 hours (see Motortechnische Zeitschrift, 1994, 55, 214-218).

The catalytically active particulate filters were then tested in theaged state at an engine test bench in the so-called “light-off test” andin the “lambda sweep test.” In the light-off test, the light-offperformance is determined in the case of a stoichiometric exhaust gascomposition with a constant average air ratio λ (λ=0.999 with 3.4%amplitude).

Table 1 below contains the temperatures T₅₀ at which 50% of each of theconsidered components is converted.

TABLE 1 T₅₀ HC T₅₀ CO T₅₀ NOx stoichiometric stoichiometricstoichiometric VGPF1 418 430 432 GPF1 377 384 387

The dynamic conversion behavior of the particulate filters wasdetermined in a lambda sweep test in a range from λ=0.99-1.01 at aconstant temperature of 510° C. The amplitude of λ in this case amountedto ±6.8%. Table 2 shows the conversion at the intersection of the CO andNOx conversion curves, along with the associated HC conversion of theaged particulate filters.

TABLE 2 CO/NOx conversion at the HC conversion at λ of the point ofintersection CO/NOx point of intersection VGPF1 79% 94% GPF1 83% 95%

The particulate filter GPF1 according to the invention shows a markedimprovement in light-off performance and dynamic CO/NOx conversion inthe aged state compared with VGPF1.

COMPARATIVE EXAMPLE 2

a) Application of the in-Wall Coating:

Aluminum oxide stabilized with lanthanum oxide was suspended in waterwith a first oxygen storage component which comprised 40% by weightcerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxideand a second oxygen storage component which comprised 24% by weightcerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Bothoxygen storage components were used in equal parts. The weight ratio ofaluminum oxide and oxygen storage component was 30:70. The suspensionthus obtained was then mixed with a palladium nitrate solution and arhodium nitrate solution under constant stirring. The resulting coatingsuspension was used directly for coating a commercially availablewall-flow filter substrate, wherein the coating was introduced into theporous filter wall over 100% of the substrate length. The total load ofthis filter amounted to 75 g/l; the noble metal load amounted to 0.71g/l with a palladium to rhodium ratio of 3:1. The coated filter thusobtained was dried and then calcined.

b) Coating the Input Channels

Aluminum oxide stabilized with lanthanum oxide was suspended in waterwith an oxygen storage component which comprised 24% by weight ceriumoxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weightratio of aluminum oxide and oxygen storage component was 56:44. Thesuspension thus obtained was then mixed with a palladium nitratesolution and a rhodium nitrate solution under constant stirring. Theresulting coating suspension was used directly to coat the wall-flowfilter substrate obtained under a), wherein the filter walls of thesubstrate were coated in the input channels to a length of 25% of thefilter length. The load of the input channel amounted to 50 g/l; thenoble metal load amounted to 2.12 g/l with a ratio of palladium torhodium of 5:1. The coated filter thus obtained was dried and thencalcined.

c) Coating the Output Channels

Aluminum oxide stabilized with lanthanum oxide was suspended in waterwith an oxygen storage component which comprised 24% by weight ceriumoxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weightratio of aluminum oxide and oxygen storage component was 56:44. Thesuspension thus obtained was then mixed with a palladium nitratesolution and a rhodium nitrate solution under constant stirring. Theresulting coating suspension was used directly to coat the wall-flowfilter substrate obtained under b), wherein the filter walls of thesubstrate were coated in the output channels to a length of 25% of thefilter length. The load of the output channel amounted to 50 g/l; thenoble metal load amounted to 2.12 g/l with a ratio of palladium torhodium of 5:1. The coated filter thus obtained was dried and thencalcined. The total load of this filter thus amounted to 100 g/l; thetotal noble metal load amounted to 1.77 g/l with a ratio of palladium torhodium of 4:1. It is hereinafter referred to as VGPF2.

EXAMPLE 2

a) Application of the in-Wall Coating:

Aluminum oxide stabilized with lanthanum oxide was suspended in waterwith a first oxygen storage component which comprised 40% by weightcerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxideand a second oxygen storage component which comprised 24% by weightcerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Bothoxygen storage components were used in equal parts. The weight ratio ofaluminum oxide and oxygen storage component was 30:70. The suspensionthus obtained was then mixed with a palladium nitrate solution and arhodium nitrate solution under constant stirring. The resulting coatingsuspension was used directly for coating a commercially availablewall-flow filter substrate, wherein the coating is introduced into theporous filter wall over 100% of the substrate length. The load of thisfilter amounted to 50 g/l; the noble metal load amounted to 0.71 g/lwith a ratio of palladium to rhodium of 3:1. The coated filter thusobtained was dried and then calcined.

b) Coating the Input Channels

Aluminum oxide stabilized with lanthanum oxide was suspended in waterwith an oxygen storage component which comprised 24% by weight ceriumoxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weightratio of aluminum oxide and oxygen storage component was 56:44. Thesuspension thus obtained was then mixed with a palladium nitratesolution and a rhodium nitrate solution under constant stirring. Theresulting coating suspension was used directly to coat the wall-flowfilter substrate obtained under a), wherein the filter walls of thesubstrate were coated in the input channels to a length of 60% of thefilter length. The load of the input channel amounted to 83.3 g/l; thenoble metal load amounted to 1.77 g/l with a ratio of palladium torhodium of 42:8. The coated filter thus obtained was dried and thencalcined. The total load of this filter thus amounted to 100 g/l; thetotal noble metal load amounted to 1.77 g/l with a ratio of palladium torhodium of 4:1. It is hereinafter referred to as GPF2

Catalytic Characterization

The particulate filters VGPF2 and GPF2 were aged together in an enginetest bench aging process. This aging process consists of an overrun fuelcut-off aging process with an exhaust gas temperature of 950° C. beforethe catalyst inlet (maximum bed temperature of 1030° C.). The aging timewas 58 hours (see Motortechnische Zeitschrift, 1994, 55, 214-218).

The catalytically active particulate filters were then tested in theaged state at an engine test bench in the so-called “light-off test” andin the “lambda sweep test.” In the light-off test, the light-offperformance is determined in the case of a stoichiometric exhaust gascomposition with a constant average air ratio λ (λ=0.999 with 3.4%amplitude).

Table 3 below contains the temperatures T₅₀ at which 50% of each of theconsidered components is converted.

TABLE 3 T₅₀ HC T₅₀ CO T₅₀ NOx stoichiometric stoichiometricstoichiometric VGPF2 356 360 365 GPF2 351 356 359

The dynamic conversion behavior of the particulate filters wasdetermined in a lambda sweep test in a range from λ=0.99-1.01 at aconstant temperature of 510° C. The amplitude of λ in this case amountedto ±6.8%. Table 4 shows the conversion at the intersection of the CO andNOx conversion curves, along with the associated HC conversion of theaged particulate filters.

TABLE 4 CO/NOx conversion at the HC conversion at λ of the point ofintersection CO/NOx point of intersection VGPF2 79% 96% GPF2 86% 97%

The particulate filter GPF2 according to the invention shows a markedimprovement in light-off performance and dynamic CO/NOx conversion inthe aged state compared with VGPF2.

COMPARATIVE EXAMPLE 3

a) Application of the in-Wall Coating:

Aluminum oxide stabilized with lanthanum oxide was suspended in waterwith a first oxygen storage component which comprised 40% by weightcerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxideand a second oxygen storage component which comprised 24% by weightcerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Bothoxygen storage components were used in equal parts. The weight ratio ofaluminum oxide and oxygen storage component was 30:70. The suspensionthus obtained was then mixed with a palladium nitrate solution and arhodium nitrate solution under constant stirring. The resulting coatingsuspension was used directly for coating a commercially availablewall-flow filter substrate, wherein the coating was introduced into theporous filter wall over 100% of the substrate length. The total load ofthis filter amounted to 100 g/l; the noble metal load amounted to 2.60g/l with a palladium to rhodium ratio of 60:13.75. The coated filterthus obtained was dried and then calcined.

b) Coating the Input Channels

Aluminum oxide stabilized with lanthanum oxide was suspended in waterwith an oxygen storage component which comprised 40% by weight ceriumoxide, zirconium oxide, lanthanum oxide and praseodymium oxide. Theweight ratio of aluminum oxide and oxygen storage component was 50:50.The suspension thus obtained was then mixed with a palladium nitratesolution and a rhodium nitrate solution under constant stirring. Theresulting coating suspension was used directly to coat the wall-flowfilter substrate obtained under a), wherein the filter walls of thesubstrate were coated in the input channels to a length of 25% of thefilter length. The load of the input channel amounted to 58 g/l; thenoble metal load amounted to 2.30 g/l with a ratio of palladium torhodium of 10:3. The coated filter thus obtained was dried and thencalcined.

c) Coating the Output Channels

Aluminum oxide stabilized with lanthanum oxide was suspended in waterwith an oxygen storage component which comprised 24% by weight ceriumoxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weightratio of aluminum oxide and oxygen storage component was 56:44. Thesuspension thus obtained was then mixed with a palladium nitratesolution and a rhodium nitrate solution under constant stirring. Theresulting coating suspension was used directly to coat the wall-flowfilter substrate obtained under b), wherein the filter walls of thesubstrate were coated in the output channels to a length of 25% of thefilter length. The load of the outlet channel amounted to 59 g/l; thenoble metal load amounted to 1.06 g/l with a ratio of palladium torhodium of 1:2. The coated filter thus obtained was dried and thencalcined. The total load of this filter thus amounted to 130 g/l; thetotal noble metal load amounted to 3.44 g/l with a ratio of palladium torhodium of 10:3. It is hereinafter referred to as VGPF3.

EXAMPLE 3

a) Application of the in-Wall Coating:

Aluminum oxide stabilized with lanthanum oxide was suspended in waterwith a first oxygen storage component which comprised 40% by weightcerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxideand a second oxygen storage component which comprised 24% by weightcerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Bothoxygen storage components were used in equal parts. The weight ratio ofaluminum oxide and oxygen storage component was 30:70. The suspensionthus obtained was then mixed with a palladium nitrate solution and arhodium nitrate solution under constant stirring. The resulting coatingsuspension was used directly for coating a commercially availablewall-flow filter substrate, wherein the coating was introduced into theporous filter wall over 100% of the substrate length. The load of thisfilter amounted to 100 g/l; the noble metal load amounted to 2.07 g/lwith a ratio of palladium to rhodium of 45:13.5. The coated filter thusobtained was dried and then calcined.

b) Coating the Input Channels

Aluminum oxide stabilized with lanthanum oxide was suspended in waterwith an oxygen storage component which comprised 24% by weight ceriumoxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weightratio of aluminum oxide and oxygen storage component was 56:44. Thesuspension thus obtained was then mixed with a palladium nitratesolution and a rhodium nitrate solution under constant stirring. Theresulting coating suspension was used directly to coat the wall-flowfilter substrate obtained under a), wherein the filter walls of thesubstrate were coated in the input channels to a length of 60% of thefilter length. The load of the input channel amounted to 80 g/l; thenoble metal load amounted to 2.30 g/l with a ratio of palladium torhodium of 10:3. The coated filter thus obtained was dried and thencalcined. The total load of this filter thus amounted to 148 g/l; thetotal noble metal load amounted to 3.44 g/with a ratio of palladium torhodium of 10:3. It is hereinafter referred to as GPF3.

Catalytic Characterization

The particulate filters VGPF3 and GPF3 were aged together in an enginetest bench aging process. This aging process consists of an overrun fuelcut-off aging process with an exhaust gas temperature of 950° C. beforethe catalyst inlet (maximum bed temperature of 1030° C.). The aging timewas 76 hours (see Motortechnische Zeitschrift, 1994, 55, 214-218).

The catalytically active particulate filters were then tested in theaged state at an engine test bench in the so-called “light-off test” andin the “lambda sweep test.” In the light-off test, the light-offperformance is determined in the case of a stoichiometric exhaust gascomposition with a constant average air ratio λ (λ=0.999 with ±3.4%amplitude).

Table 5 below contains the temperatures T₅₀ at which 50% of each of theconsidered components is converted.

TABLE 5 T₅₀ HC T₅₀ CO T₅₀ NOx stoichiometric stoichiometricstoichiometric VGPF3 363 374 371 GPF3 341 345 340

The dynamic conversion behavior of the particulate filters wasdetermined in a lambda sweep test in a range from λ=0.99-1.01 at aconstant temperature of 510° C. The amplitude of λ in this case amountedto ±6.8%. Table 6 shows the conversion at the intersection of the CO andNOx conversion curves, along with the associated HC conversion of theaged particulate filters.

TABLE 6 CO/NOx conversion at the HC conversion at λ of the point ofintersection CO/NOx point of intersection VGPF3 83% 97% GPF3 90% 98%

The particulate filter GPF3 according to the invention shows a markedimprovement in light-off performance and dynamic CO/NOx conversion inthe aged state compared with VGPF3.

It was furthermore systematically investigated what the main effectsresponsible for the lowest possible exhaust back pressure are. In doingso, various filters with different zone lengths (factor A) and washcoatlayer thicknesses (factor B) were prepared and compared with oneanother. All filters had the same total washcoat load and the same noblemetal content.

TABLE 7 Factor Name Unit Min Max A Zone length % 30 60 B Washcoatthickness g/l 50 80

The statistical evaluation shows that it is particularly advantageous todistribute the washcoat on as large a surface as possible on the filterwalls with a resultant low layer thickness instead of covering only asmall surface with a high layer thickness, since a high layer thicknessis to be regarded as the main cause of a high exhaust back pressure(FIG. 2). In addition, the particulate filters were aged together in anengine test bench aging process. This aging process consists of anoverrun fuel cut-off aging process with an exhaust gas temperature of950° C. before the catalyst inlet (maximum bed temperature of 1030° C.).The aging time was 19 hours (see Motortechnische Zeitschrift, 1994, 55,214-218).

The catalytically active particulate filters were then tested in theaged state at an engine test bench in the so-called “lambda sweep test.”Surprisingly, the statistical evaluation of the test results also showsa significant advantage in the lambda sweep test if the catalyticcoating is applied with a low layer thickness to as large a surface aspossible (FIG. 3).

1. A particulate filter for removing particles, carbon monoxide,hydrocarbons and nitrogen oxides from the exhaust gas of combustionengines fueled by stoichiometric air-fuel mixtures, which filtercomprises a wall-flow filter of length L and two different coatings Yand Z, wherein the wall-flow filter comprises channels E and A thatextend in parallel between a first and a second end of the wall-flowfilter and are separated by porous walls which form the surfaces O_(E)and O_(A), respectively, and wherein the channels E are closed at thesecond end and the channels A are closed at the first end, characterizedin that coating Y is located in the channels E on the surfaces O_(E) andextends from the first end of the wall-flow filter over a length of 51to 90% of the length L, and coating Z is located in the porous walls andextends from the second end of the wall-flow filter over a length of 60to 100% of the length L.
 2. The particulate filter in accordance withclaim 1, characterized in that the coating Y extends from the first endof the wall-flow filter over 51 to 80% of the length L of the wall-flowfilter.
 3. The particulate filter in accordance with claim 2,characterized in that the coating Y extends from the first end of thewall-flow filter over 57 to 65% of the length L of the wall-flow filter.4. The particulate filter in accordance with claim 1, characterized inthat the coatings Y and Z have a thickness between 5-250 μm.
 5. Theparticulate filter in accordance with claim 1, characterized in thateach of the coatings Y and Z contains one or more noble metals fixed toone or more substrate materials, and one or more oxygen storagecomponents.
 6. The particulate filter in accordance with claim 5,characterized in that each of the coatings Y and Z contains the noblemetals platinum, palladium and/or rhodium.
 7. The particulate filter inaccordance with claim 5, characterized in that each of the coatings Yand Z contains the noble metals palladium, rhodium or palladium andrhodium.
 8. The particulate filter in accordance with claim 4,characterized in that the substrate materials for the noble metals aremetal oxides with a BET surface area of 30 to 250 m²/g (determinedaccording to DIN 66132—newest version on the date of application). 9.The particulate filter in accordance with claim 5, characterized in thatthe substrate materials for the noble metals are selected from theseries consisting of aluminum oxide, doped aluminum oxide, siliconoxide, titanium dioxide and mixed oxides of one or more of these. 10.The particulate filter in accordance with claim 5, characterized in thatthe coatings Y and Z contain a cerium/zirconium/rare earth metal mixedoxide as oxygen storage component.
 11. The particulate filter inaccordance with claim 10, characterized in that thecerium/zirconium/rare earth metal mixed oxides contain lanthanum oxide,yttrium oxide, praseodymium oxide, neodymium oxide and/or samarium oxideas rare earth metal oxide.
 12. The particulate filter in accordance withclaim 10, characterized in that the cerium/zirconium/rare earth metalmixed oxides contain lanthanum oxide and yttrium oxide, yttrium oxideand praseodymium oxide or lanthanum oxide and praseodymium oxide as rareearth metal oxide.
 13. The particulate filter in accordance with claim5, characterized in that the coatings Y and Z both compriselanthanum-stabilized aluminum oxide, palladium, rhodium or palladium andrhodium and an oxygen storage component comprising a zirconium oxide,cerium oxide, yttrium oxide and lanthanum oxide and/or a zirconiumoxide, cerium oxide, praseodymium oxide and lanthanum oxide.
 14. Theparticulate filter in accordance with claim 1 comprising a wall-flowfilter of length L and two different coatings Y and Z, wherein thewall-flow filter comprises channels E and A that extend in parallelbetween a first and a second end of the wall-flow filter and areseparated by porous walls which form the surfaces O_(E) and O_(A),respectively, and wherein the channels E are closed at the second endand the channels A are closed at the first end, characterized in thatCoating Y is located in the channels E on the surfaces O_(E) and extendsfrom the first end of the wall-flow filter over 57 to 65% of the lengthL and contains aluminum oxide in an amount of 35 to 60% by weight, basedon the total weight of the coating Y, palladium, rhodium or palladiumand rhodium and an oxygen storage component in an amount of 40 to 50% byweight, based on the total weight of the coating Y, wherein the oxygenstorage component comprises zirconium oxide, cerium oxide, lanthanumoxide and yttrium oxide or zirconium oxide, cerium oxide, lanthanumoxide and praseodymium oxide, and Coating Z is located in the porouswalls and extends from the second end of the wall-flow filter over 60 to100% of the length L and contains aluminum oxide in an amount of 25 to50% by weight, based on the total weight of the coating, palladium,rhodium or palladium and rhodium and two oxygen storage components in atotal amount of 50 to 80% by weight, based on the total weight of thecoating Z, wherein one oxygen storage component contains zirconiumoxide, cerium oxide, lanthanum oxide and yttrium oxide and the othercontains zirconium oxide, cerium oxide, lanthanum oxide and praseodymiumoxide.
 15. A method for removing particles, carbon monoxide,hydrocarbons, and nitrogen oxides from the exhaust gas of combustionengines fueled by a stoichiometric air-fuel mixture, characterized inthat the exhaust gas is conducted through a particulate filter inaccordance with claim 1.